North Korea could test a powerful nuclear weapon over the Pacific Ocean in response to US President Donald Trump's threats of military action, the country's foreign minister has warned.

Ri Yong Ho spoke to reporters in New York shortly after North Korean leader Kim Jong Un made an unprecedented televised statement, accusing Trump of being "mentally deranged."

The forceful rhetoric from Pyongyang came after Trump threatened to"totally destroy" North Korea in a speech to the UN General Assembly on Tuesday. Trump tweeted Friday that Kim was "obviously a madman" who would be "tested like never before."

In a rare direct statement delivered straight to camera, Kim said that Trump would "pay dearly" for the threats, and that North Korea "will consider with seriousness exercising of a corresponding, highest level of hard-line countermeasure in history."

"I am now thinking hard about what response he could have expected when he allowed such eccentric words to trip off his tongue," Kim said. "I will surely and definitely tame the mentally deranged US dotard with fire."

Kim said Trump's comments were reflective of "mentally deranged behavior."

Hours later, Kim's foreign minister told reporters in New York that Pyongyang could launch a nuclear missile test in response. "This could probably mean the strongest hydrogen bomb test over the Pacific Ocean. Regarding which measures to take, I don't really know since it is what Kim Jong Un does," said Ri.

Japan's defense minister Itsunori Onodera said the country must ready itself for the sudden escalation in tensions and be prepared for a missile launch.

In response, Trump said on Twitter: "Kim Jong Un of North Korea, who is obviously a madman who doesn't mind killing or starving his people, will be tested like never before!"

First-person first?

The phrase "highest level of hard-line countermeasure in history" could be considered an escalation in the choice of language used, said Vipin Narang, a professor of political science at MIT and expert on deterrence and nuclear policy.

"This is clearly trying to coerce the US into playing ball," Narang told CNN.

In his first address to the United Nations as US President, Trump said that the US was ready to "totally destroy" North Korea if it was forced to defend its allies, a warning seen as unprecedented for a US president delivering an address to the world's leaders and top diplomats.

Responding to the speech, Kim said Trump's comments amounted to an insult. "I'd like to advise Trump to exercise prudence in selecting words and to be considerate of whom he speaks to when making a speech in front of the world," Kim said.

A handful of North Korea analysts believe Kim's response -- the first time he has ever released a first-person statement -- could show how personally the young leader took Trump's speech.

"This is unprecedented, as far as we can tell," Narang said. "It's written by him, it's signed by him ... He was clearly offended by the speech, and what concerns me most is the response he says he is considering."

North Korea was scheduled to speak at the UN General Assembly Friday night, but dropped off of its planned roster spot. The country could still get a slot at another time.

More sanctions

The White House, meanwhile, took the another step in its so-called "peaceful pressure" campaign to rein in Pyongyang's nuclear program, expanding sanctions on North Korea and those who do business with the country.

Though the majority of North Korea's imports come from China, Treasury Secretary Steve Mnuchin said "This action is directed at everyone" and the steps are "in no way specifically directed at China."

The executive order Trump inked just ahead of the lunch enhances Treasury Department authorities to target individuals who provide goods, services or technology to North Korea, Trump said. He said the order would also allow the US to identify new industries -- including textiles, fishing and manufacturing -- as potential targets for future actions.